Coming now to a consideration of the complete selective-signaling lock-out systems, wherein the selection of the party and the locking out of the others are both inherent features, a single example of the step-by-step, and of the broken-line selective lock-out systems will be discussed.

Step-by-Step System. The so-called K.B. system, manufactured by the Dayton Telephone Lock-out Manufacturing Company of Dayton, Ohio, operates on the step-by-step principle. The essential feature of the subscriber's telephone equipment in this system is the step-by-step actuating mechanism which performs also the functions of a relay. This device consists of an electromagnet having two cores, with a permanent polarizing magnet therebetween, the arrangement in this respect being the same as in an ordinary polarized bell. The armature of this magnet works a rocker arm, which, besides stepping the selector segment around, also, under certain conditions, closes the bell circuit and the talking circuit, as will be described.

Fig. 189. K.B. Lock-Out System
[View full size illustration.]

Referring first to Fig. 189, which shows in simplified form a four-station K.B. lock-out line, the electromagnet is shown at 1 and the rocker arm at 2. The ratchet 3 in this case is not a complete wheel but rather a segment thereof, and it is provided with a series of notches of different depths. It is obvious that the depth of the notches will determine the degree of movement which the upper end of the rocker arm may have toward the left, this being dependent on the extent to which the pawl 6 is permitted to enter into the segment. The first or normal notch, i.e., the top notch, is always of such a depth that it will allow the rocker-arm lever 2 to engage the contact lever 4, but will not permit the rocker arm to swing far enough to the left to cause that contact to engage the bell contact 5. As will be shown later, the condition for the talking circuit to be closed is that the rocker arm 2 shall rest against the contact 4; and from this we see that the normal notch of each of the segments 3 is of such a depth as to allow the talking circuit at each station to be closed. The next notch, i.e., the second one in each disk, is always shallow, as are all of the other notches except one. A deep notch is placed on each disk anywhere from the third to the next to the last on the segment. This deep notch is called the selective notch, and it is the one that allows of contact being made with the ringer circuit of that station when the pawl 6 drops into it. The position of this notch differs on all of the segments on a line, and obviously, therefore, the ringer circuit at any station may be closed to the exclusion of all the others by stepping all of the segments in unison until the deep notch on the segment of the desired station lies opposite to the pawl 6, which will permit the rocker arm 2 to swing so far to the left as to close not only the circuit between 2 and 4, but also between 2, 4, and 5. In this position the talking and the ringing circuits are both closed.

The position of the deepest notch, i.e., the selective notch, on the circumference of the segment at any station depends upon the number of that station; thus, the segment of Station 4 will have a deep notch in the sixth position; the segment for Station 9 will have a deep notch in the eleventh position; the segment for any station will have a deep notch in the position corresponding to the number of that station plus two.

From what has been said, therefore, it is evident that the first, or normal, notch on each segment is of such a depth as to allow the moving pawl 6 to fall to such a depth in the segment as to permit the rocker arm 2 to close the talking circuit only. All of the other notches, except one, are comparatively shallow, and while they permit the moving pawl 6 under the influence of the rocker arm 2 to move the segment 3, yet they do not permit the rocker arm 2 to move so far to the left as to close even the talking circuit. The exception is the deep notch, or selective notch, which is of such depth as to permit the pawl 6 to fall so far into the segment as to allow the rocker arm 2 to close both the talking and the ringing circuits. Besides the moving pawl 6 there is a detent pawl 7. This always holds the segment 3 in the position to which it has been last moved by the moving pawl 6.

The actuating magnet 1, as has been stated, is polarized and when energized by currents in one direction, the rocker arm moves the pawl 6 so as to step the segment one notch. When this relay is energized by current in the opposite direction, the operation is such that both the moving pawl 6 and the detent pawl 7 will be pulled away from the segment, thus allowing the segment to return to its normal position by gravity. This is accomplished by the following mechanism: An armature stop is pivoted upon the face of the rocker arm so as to swing in a plane parallel to the pole faces of the relay, and is adapted, when the relay is actuated by selective impulses of one polarity, to be pulled towards one of the pole faces where it acts, through impact with a plate attached to the pole face of the relay, as a limiting means for the motion of the rocker arm when the rocker arm is actuated by the magnet. When, however, the relay is energized by current in the opposite direction, as on a releasing impulse, the armature stop swings upon its pivot towards the opposite pole face, in which position the lug on the end of the armature stop registers with a hole in the plate on the relay, thus allowing the full motion of the rocker arm when it is attracted by the magnet. This motion of the rocker arm withdraws the detent pawl from engagement with the segment as well as the moving pawl, and thereby permits the segment to return to its normal position. As will be seen from Fig. 189, each of the relay magnets 1 is permanently bridged across the two limbs of the line.

Each station is provided with a push button, not shown, by means of which the subscriber who makes a call may prevent the rocker arm of his instrument from being actuated while selective impulses are being sent over the line. The purpose of this is to enable one party to make a call for another on the same line, depressing his push button while the operator is selecting and ringing the called party. The segment at his own station, therefore, remains in its normal position, in which position, as we have already seen, his talking circuit is closed; all of the other segments are, however, stepped up until the ringing and talking circuits of the desired station are in proper position, at which time ringing current is sent over the line. The segments in Fig. 189, except at Station C, are shown as having been stepped up to the sixth position, which corresponds to the ringing position of the fourth station, or Station D. The condition shown in this figure corresponds to that in which the subscriber at Station C originated the call and pressed his button, thus retaining his own segment in its normal position so that the talking circuits would be established with Station D.